1. Field of the Invention
The invention relates to a helical gear having a spindle which is secured in the axial direction and has a spindle axis, and a spindle nut which is connected to a slide which can be driven and is guided in the axial direction, a joint being provided between the spindle nut and the slide in order to compensate for relative transverse movements and tilting between the spindle nut and slide, said joint being formed symmetrically to the spindle axis and having two joint axes which are perpendicular to each other, intersect on the spindle axis and are assigned at one end to the transverse movement of a bearing plate, which is connected to the spindle nut, and at the other end to the transverse movement of a bearing plate, which is connected to the slide, and said joint containing an intermediate plate via which the bearing plates are connected pivotably to each other in order to compensate for any tilting of their joint axes.
2. Description of the Related Art
Helical gears of this type are known and are described, for example, in The textbook "Korstruktionselemente der Feinmechanik" [Structural elements in precision mechanics], edited by Werner Krause, Hanser publishers (1989), pp. 709 ff. The spindle is rotatable, but is not displaced in the axial direction. The spindle nut is retained in a longitudinal guide parallel to the axis of the spindle and is therefore not rotatable. Rotation of the spindle is therefore converted into a sliding movement of the spindle nut. The object carrier which is to be driven is connected to the spindle nut.
With helical gears it is possible, through the selection of sufficiently long spindles, to obtain drives over very long displacement paths. Generally, not too much has to be demanded as concerns the quality of the screw thread of the spindle although, for example, hardened spindles having a ground screw thread for great thread precision are preferable. Play between the screw threads of the spindle nut and the spindle can be compensated for by the spindle nut undergoing a division with the parts spring-loaded against one another. The axes of long spindles are generally not exactly linear. Particularly in the case of thin spindles, they can either be slightly bent or even corrugated. Since they are only mounted in their end regions, the spindle nut therefore executes slight upward and downward movements and also lateral movements along its displacement path. Such eccentricity in the concentric running of the spindle is also produced if the axis of a driving motor and the spindle axis are not aligned with each other.
If, on the other hand, the axial guiding of the slide defines the displacement path vertically and laterally, there occur, in particular in the case of very precise slide guides, distortions between the driven nut and the slide. These distortions can be further reinforced if the bearings of the spindle are not arranged for alignment exactly parallel to the displacement path. Distortions of this type considerably impair the running characteristics and the positioning precision of the slide.
U.S. Pat. No. 5,392,662 discloses a helical gear in which the above mentioned error effects are compensated for by a joint fitted between the spindle nut and slide. The joint consists of a contact-pressure plate which is connected to the spindle nut, an intermediate plate and a contact-pressure plate which is connected to the slide. Each of the bearing plates is connected to the intermediate plate via two spring clips. The two spring clips of a bearing plate lie parallel to each other on opposite sides of the spindle axis, the two pairs of spring clips being arranged perpendicular to each other. The spring clips are fastened to the bearing plates by their rounded part and to the intermediate plate by their foot parts. As a result, each of the bearing plates is guided, with regard to a transverse movement, by a spring parallelogram and, with regard to tilting, can be rotated about an axis. The joint is configured symmetrically to the spindle axis and contains a non-positive connection between the driving and driven system. In this case, linear compression or deformation of the spring clips in the case of resistance between the spindle drive and slide guide or in the case of relatively high driving frequencies cannot be ruled out. The deflection of the spring clips during the transverse movement produces a force reaction on the spindle, said reaction increasing with the deflection.